Ich does not go dormant. It has a defined life cycle. It's not in all water, or all tanks.
Google, "Marine ich Myths and Facts" And you'll find whole lot of information, from information on the life-form, to how to treat it.
Unfortunately the only 100% Method is to place all fish into quarantine and treat them outside your main display. Leaving the main display empty[fallow] for 72 days. This will starve the parasite in the display, ridding you of it.
You can treat the fish with your favorite method of ich treatment. I personally use Copper. You can use hyposalinity if you want, just make sure you never combine Hyposalinity and copper.